Starting with Activiti Designer 5.10 you have disabled the form property section for Timer Start Events (... disabled for all start events holding some kind of event definition).
Of course a message start event or a signal start event is something completely different (and I'm allowed to define such a start event additionally to a non start event) - but in case of a timer start I'm missing the former ability to edit the form properties.

I know that there are workarounds and other ways to start a process. But this kind of definition is very usefull in case you have:

a lot of processes working with only simple start parameters. Instead of defining a complete start form for each of these process I can use some kind of automatic rendering and at the same time have my start form properties documented directly within the process.

processes of this kind which should run on timer start as well as manually.